As a structure of ball joint for performing such function a metal sintering material has been used until now on the seat on the load-supporting side which supports the ball head, or synthetic resin materials such as polyethylene, polyurethane, or polyamide having elasticity and wear resistance have been used.
The former, however, has defects such as a torque increase under load and a seizuring in use, in contrast to its larger mechanical strength, while the latter may bring about a cold flow of the resin material, flowing out from the opening at which the ball stud extends.